This invention relates generally to an improved metal halide lamp for stabilized operation at acoustic frequencies and more particularly to an asymmetric electrode configuration in such type lamp providing an improved light source when so operated.
Various metal halide discharge lamps commonly employ a fused quartz arc tube as the light source by reason of the refractory nature and optical transparency of this vitreous ceramic material. In such type lamps the arc tube generally comprises a sealed envelope formed with fused quartz tubing having spaced apart discharge electrodes hermetically sealed at the ends thereof to provide an arc gap therebetween which is centered in the envelope cavity. A common arc tube shape employed for automotive vehicle applications has a double-ended configuration formed by neck portions at each end of a bulbous spherically shaped central portion with the symmetrically oriented electrode means being hermetically sealed in the neck portions. The sealed arc tube further contains a fill of various metal substances which become vaporized during lamp operation to include sodium, mercury and metal halides along with one or more rare inert gases such as krypton, argon and xenon. The symmetrically constructed metal vapor discharge lamps can be operated with power sources which deliver time varying current. Lamp operation with current varying at acoustic frequencies has been found particularly suitable for a variety of end-product applications such as reflector, spot and flood lamps or vehicle headlamps.
In the above referenced Ser. No. 07/579,129 application, now U.S. Pat. No. 5,121,034 means are provided to largely overcome various severe problems encountered with operation of such type lamps due to adverse acoustic resonance effects. The general nature of these problems is therein attributed to acoustic wave generation within the arc tube fill by lamp operation at current frequencies higher than the usual main frequency (60 Hz), for instance in the frequency range from 500 Hz to 1 MHz. Such acoustic wave patterns within the lamp envelope occasion changes in position of the discharge arc or its glowing aureole or plume, changes in the color of the emitted light and/or unstable discharge arcs resulting in flicker and sometimes extinction of the discharge arc. The problems are further said to be largely avoided by a method of lamp operation which forces a current having an alternating component causing instantaneous variation in input power across the discharge electrode gap with variations in power being at a frequency selected in preferred bands within the range from about 1 KHz up to about 1 MHz, the band being one in which acoustic resonance excites arc-straightening modes which reduce the effects of gravity-induced convection in the arc tube fill. Various embodiments for operating these lamps in such improved manner are also disclosed in the aforesaid co-pending application. One embodiment selects the preferred band so that acoustic resonance excites a mode effective to reduce gravity-induced bowing of the discharge, lower the hot spot temperature, and raise the cold spot temperature during lamp operation. Another embodiment employs alternating current having a waveshape providing time-fluctuation of input power at the selected frequency. In a further embodiment, the alternating component of the current through the lamp at the selected frequency is frequency-modulated in order to broaden the width of the band of frequencies in which a straight and stable arc is achieved. Still further embodiments are also disclosed employing a horizontal arc gap orientation parallel to the major or center lamp axis and which can further include employment of xenon as a radiation-emitting gas in the arc tube fill. In one such further lamp embodiment, the electrodes define a horizontal arc gap parallel to the major lamp axis, the radiation-emitting gas is xenon in a quantity exerting a partial pressure comparable to that of the mercury included in the arc tube fill, and wherein the arc tube fill further includes sodium being provided as a halide.
While acoustic operation of these type lamps in the foregoing manner is recognized to improve lamp performance, the symmetric lamp constructions remain sensitive to spatial orientation when being so operated. More particularly, such lamp constructions are frequently de-rated for non-vertical operation as well as further having occasioned special arc tube configurations for horizontal lamp operation. In the latter respect, bent arc tubes have been developed along with having displaced the discharge electrodes from the longitudinal center axis of the arc tube and with still further magnetic field compensation having been employed to improve performance of non-vertically operated lamps. Despite all such improvements directed primarily to providing a straighter arc discharge during non-vertical lamp operation in order to avoid deleterious heat convection effects, some asymmetric heating of the arc tube walls still takes place. Undesirable color separation of the arc discharge also occurs in the symmetric lamp constructions whereby the core of the principal discharge is observed to have a blue color while the edges of the discharge are observed to have a discernible red color. While acoustic lamp operation has also been observed to reduce color separation in the symmetric arc tube constructions, such improvement is found to often still remain transitory due to difficulty in maintaining a stable arc condition.
Accordingly, it is an object of the present invention to provide an improve metal halide lamp particularly adapted for lamp operation at acoustic frequencies and which exhibits less color separation in the principal arc discharge region.
It is a further object of the present invention to provide a xenon-metal halide lamp particularly suited for automotive applications.
It is another object of the present invention to provide a xenon-metal halide lamp less dependent upon spatial orientation when being operated.
Still a further object of the present invention is to provide an automotive headlamp employing such improved lamp construction for its light source.
These and other objects of the present invention will become apparent upon considering the following more detailed description.